The present invention relates to unsaturated polyester resin compositions containing thermoplastic additives. The unsaturated polyester resin compositions have improved viscosity control, phase stability, and tack.
It has long been known in the art of molding thermoset unsaturated polyester compositions that prepolymers of dihydric alcohols and unsaturated dicarboxylic acids may be copolymerized with unsaturated monomers to form molded articles. It has also been known that additional, nonreacting thermoplastic polymers can be added to control the shrinkage that occurs during cure of these resins. These thermoplastics polymers are well-known as "low-profile additives" due to the fact that a "low profile", or highly smooth surface appearance is achieved when these "low profile additives" are used in conjunction with thermoset unsaturated polyester compositions. The unsaturated polyester resin compositions which contain low profile additives are known as "low profile resins" and have improved surface smoothness since the low-profile additives reduce or eliminate the amount of shrinkage that occurs during the cure of the unsaturated polyester resin composition.
Two major systems of low-profile resins currently in use are the one-pack and two-pack systems. The major characteristic that distinquishes the two systems is the degree of compatibility of the low-profile thermoplastic polymer with the unsaturated polyester and styrene. In two-pack systems, if the low-profile additive is incompatible with the unsaturated polyester and monomer copolymer, the resin must be mixed immediately before use in sheet molding composition (SMC) formulations. In one-pack systems, the low-profile additive, unsaturated polyester, and monomer are mutually compatible to the point that no gross phase separation occurs. In both systems, it is the ability of the low-profile resins to compensate for shrinkage that makes them especially useful in SMC formulations.
Thus, shrinkage compensation is the distinguishing characteristic of low-profile resins. The ability of the low-profile resins to compensate for shrinkage is largely a result of a micro-phase separation that occurs during cure for both one- and two-pack systems. Prior to cure, the low-profile additive is at least partly soluble in the polyester/monomer solution. As the polyester/monomer mixture crosslinks and forms a copolymer, the low-profile additive and polyester/monomer copolymer become increasingly less compatible and a two-phase (domain-matrix) type morphology results. This micro-phase separation leads to the formation of a porous structure as the opposing internal stresses of thermal expansion and polymerization shrinkage occur. In many unsaturated polyester resin compositions the porous structure is a result of microfracturing of the curing resins which consequentially gives rise to void formation. It is also known that various unsaturated polyester resin compositions have been developed which have essentially zero shrinkage and which in fact expand upon curing.
The unsaturated polyester resin compositions are used in making sheet molding compound (SMC) formulations which often contain other ingredients such as chemical thickeners. In such SMC formulations, an alkaline material such as magnesium oxide or magnesium hydroxide is added to, for example, an uncured polyester along with fillers, glass fiber, and other standardly used materials. The alkaline material interacts with the residual acidity in the polyester and, usually, in the low-profile additive, to build viscosity. This process is referred to as maturation and usually takes several days. If a two-pack resin system is used, care has to be taken to avoid gross phase separation. After maturation is complete, the thickened SMC formulations are handable and can easily be placed into compression molds either by hand or by machine.
It has previously been established in low-profile additive technology that the acid functionality of the unsaturated polyester or low-profile additive was necessary for control of the thickening or maturation process. For example, the residual acid groups of unsaturated polyesters are one source of carboxylic acids. Thermoplastic additives, such as a polyvinylacetate copolymers, are typically copolymerized with acid functional monomers so that at least one acid functionality is incorporated along the chain of the copolymer. For example, Comstock et al. U.S. Pat. No. 3,718,714 describes a vinyl acetate copolymer that contains an average of at least one carboxyl group per molecule.
Various saturated polyesters have also been used as low-profile additives in unsaturated polyester resins. These saturated polyesters typically contain some residual acid functionality due to the remainder of unreacted carboxyl groups of the starting acids. For example, Hindersinn et al., U.S. Pat. No. 3,909,483 describes saturated polyesters having terminal carboxylic acid groups.
The use of trimelletic anhydride as a comonomer in polyester resins has been described by Chang et al., U.S. Pat. No. 4,504,619 where an unsaturated polyester containing 2,2,4-trimethylpentane diol, isophthalic acid, maleic anhydride and trimellitic anhydride was prepared.
Thomas et al. U.S. Pat. No. 4,024,111 have prepared a dianhydride by reacting 1.5 moles of ethylene glycol with three moles of trimellitic anhydride in order to avoid gellation.
Although generally the use of low-profile additives and thickening agents are known to be effective in improving the thickening characteristics of sheet molding compositions, until now it has previously not been possible to reliably control the viscosity, phase stability and tackiness of unsaturated polyester resin compositions that contain certain thermoplastic low-profile additives.